1. Field of the Invention
This invention relates to a DC-DC converter for obtaining a predetermined voltage from a direct current power source such as a battery.
2. Description of the Related Art
DC-DC converters have been used widely in systems having a battery or a direct current power supply. FIG. 10 shows an example of a DC-DC converter for use in such systems, which is a boosting type DC-DC converter. A coil 2 and a switching device 5 are connected in series to a power source 1. An anode of a diode 3 is connected to the connection point between the coil 2 and the switching device 5, and a capacitor 4 is connected between a cathode of the diode 3 and a minus terminal of the power source 1. In this DC-DC converter, when the switching device 5 is turned on, a current flows through the power source 1, the coil 2 and the switching device 5. When the switching device 5 is turned off, a current flows through the power source 1, the coil 2, the diode 3 and the capacitor 4. Energy is thereby accumulated in the capacitor 4. This operation is repeated until an output voltage becomes equal to a predetermined voltage. When the predetermined voltage is reached, this operation is stopped. Thereafter, when the output voltage is reduced to a certain level, the operation is repeated again. The output voltage is thereby maintained substantially constantly.
In this kind of DC-DC converter, switching of the switching device is determined depending upon the output voltage. Therefore, it is difficult to anticipate resulting variations in the power supply voltage related to the switching. For this reason, a filter circuit capable of filtering over a range from a low frequency to a high frequency is required. Such a filter circuit is complicated and expensive, requires a large mount space, and is therefore unsuitable for simple products and products designed to be smaller in size. If the filter circuit of a DC-DC converter provided in a camera or the like is simplified, it is possible that the automatic focusing accuracy will be seriously reduced and that a remote control signal or the like will be erroneously accepted or will not be accepted, resulting in failure to achieve necessary functions.
DC-DC-converters designed to solve this problem have been provided. That is, pulse width modulation (PWM) type DC-DC converters in which the on and off times of a switching device are changed according to the output voltage and current mode type DC-DC converters in which a coil current corresponding to the difference between the output voltage and a set target voltage is formed have been proposed and realized as products. In current mode type DC-DC converter among such DC-DC converters, variations in the power supply voltage can be limited particularly effectively and there is practically no need for a complicated expensive filter circuit, because the on-time can be controlled more linearly in current mode type DC-DC converters than in PWM type DC-DC converters. Further, current mode type DC-DC converters can have a current caused to flow through a coil according to the difference between the output current and a set target current, and can therefore avoid excessive current through the coil. Also, even if, for example, the coil is changed, only a section for detecting the coil current may be changed to achieve the desired performance while the same switching frequency and the same on time are maintained.
However, the rising rate of the output voltage from 0 V in current mode type DC-DC converters is lower than that in other conventional DC-DC converters, because of the configuration for causing a current corresponding to the difference between the output voltage and a set target voltage to flow through the coil. Current mode type DC-DC converters also have a drawback in that the output voltage is reduced if the load is increased with respect to a set target value, because the output voltage regulation with respect to variations in the load is such that the output voltage is determined at a point of balance between the load current and the current supplied from the coil.